I recommend you do NOT take any Rogue PrCs that have no sneak attack progression. Sneak attack is one of the Rogue's best abilities, so why would you want to give it up? Especially in your case, Sparhawk, since you want to be useful in combat.
I'm not sure what you mean by not wanting to be a leader, but that may be a reference to the Guild Thief PrC. The Guild Thief does get bonuses to his Leadership score, but the Leadership feat in D&D isn't necessarily about leadership. You can use it to get a useful NPC henchman or creature of some kind. If you wanted something like a shadow panther like Guenhwyvar, or an awakened monkey with Rogue levels that's a great pick pocket, you would use the Leadership feat. There are many, many possibilities with that feat.
For you, though, I'd recommend the Nightsong Enforcer, if you can get a copy. It's from Dragon Magazine, and written by Monte Cook. You get full sneak attack progression and some very useful combat abilities, i.e., it's good for a combat rogue. Some of the combat abilities include Crippling Strike, lower armor check penalties, and Blindsight. In exchange for those abilities, you only get 4 skill points per level.